Ever wondered what the "Base ISO" (or "Native ISO") of your camera is? No? Neither had I until today. If the question doesn't make sense then it may help if I describe how ISO is changed by the camera's electronics. This description will, no doubt, have the experts in the field gnashing their teeth in horror but I hope I have represented the essentials correctly.

Each of the pixels on a camera is a little photo-sensor which frees electrons as photons of light are absorbed. The process isn't anywhere near 100% efficient but the number of electrons freed is proportional to the number of photons seen and those electrons are stored at each pixel until they are read out by more electronics and an analogue to digital converter converts the resultant voltage into a number, usually with 12 or 14 bit resolution. The trick is to realise that the sensitivity/efficiency of the photo-sensor doesn't change when you change the ISO number. What does change is the amount of amplification given to the voltage from the photo-sensor which then goes to the A/D converter.

With that in mind it is obvious that, for a given light intensity on the sensor, regardless of the ISO number you dial in, the individual pixels will always take the same time get to a "full well" state, that is a state where they can't store any more electrons. Of course the resultant signal can't be properly used if you dial in a high ISO number because the subsequent voltage amplification will either clip the analogue signal or the resultant digital number will hit the 12 or 14 bit maximum (blown highlights).

So how does this help define "native ISO"? Unfortunately I haven't been able to find a proper definition so how about this? A sensor's native ISO is the same as the ISO number of a camera film which, when exposed to the same light intensity, reaches full saturation in the same time as the digital sensor reaches "full-well". By "full saturation" I mean that the photosensitive chemicals in the film have all been activated by incoming photons. A pretty sloppy definition, no doubt, but I hope it will do for the job in hand.

The reason why native ISO matters is that when you take a photograph at this ISO number the highlights will, if the picture is correctly exposed, have just resulted in an almost full-well number of electrons at each relevant pixel. This means that the statistical noise is lowest and the noise introduced by the read electronics is low in proportion to the signal. If you dial in a higher ISO than the native ISO the amplifiers have to work with an input signal which may, for really high ISO numbers, be much less than full-well (less photons equals less electrons) and so the underlying noise will be higher in proportion to the signal.

"So what," you yawn. We all know that using high ISO numbers results in more sensor noise. More surprising, perhaps, is that using an ISO number below the native ISO number also degrades the image. The reason is that you are effectively telling the camera to blow the highlights as the brightest elements of the picture will get to full-well before the exposure is over. It gets worse because the amplifiers are now set to a lower than optimal gain meaning that, as I understand it, shadow detail can also be lost into the noise threshold of the electronics.

In case you think this is academic have a look at this diagram for the Canon EOS 1Ds MkII, where halving the ISO number from 100 to 50 results in a S/N ratio equivalent to working at ISO 800!

Nikon D300 owners must also be well aware of this as the manual warns that setting ISO numbers below 200 will also degrade the image. The lesson seems clear that if you need to use an ISO number below your camera's native ISO then you should actually invest in one or more ND (neutral density) filters.

One final thought concerns why so many cameras have an ISO setting of 100 or 200 as their lowest standard ISO number. Looking at this table it would seem that, to take the case of the Nikon D300, the native ISO of the sensor is actually about 140. That gives every reason for Nikon to warn against using ISO 100 but why have a lowest recommended ISO of 200? I have no idea.

Bob.

Edit: Thread title changed to reflect use of "Base ISO" as well as "Native ISO"

Bob, I read it twice, and I don't get it. Tell me what our 40D's are supposed to have as natives. And then tell me how to find out if I am even close to natives, and if I should change up my natives, and then what about the other stuff that might have to change if the natives are not appeased. I don't even KNOW any natives.

Enlighten me. Please. You can give me a long answer, I like to read and try and figure it out..Roy, however, wants you to cut to the chase.
So? How do we do it?

A short reply to a long(ish) post.... so what is the native ISO of our cameras then Bob.Can I have a short(ish) answer.

BTW Didn't understand a word of that above

The short answer is that I don't know! Given how rapidly the S/N ratio deteriorates if ISO numbers less than the sensor's native ISO are used it's a fair bet that it's not significantly more than 100 for the 40D and may even be a little less.

I've tried Googling to find out the answer with no success. There is a thread out there which seems to indicate that ISO 160 is the correct figure but I don't buy into that for two reasons. Firstly, if the 40D native ISO were 160 then ISO 100 images would be noticeably inferior to ISO 200 images (note how aggressive Nikon is in this regard - see this table). Secondly, the tests were done with the lens cap on so whatever they are measuring isn't the effect I was describing in my initial post.

By the way, I'm sorry that my post wasn't as clear as it should have been. Maybe that graph is worth more than all the words and, unless you have a camera which allows a "Lo" ISO setting below the nominal minimum, maybe the entire post should be ignored.

Gordon, do you know if any of the body manufacturers publish the native ISOs of their sensors?

One thing I noticed is that Olympus's release of the E510 starts at ISO 100, then 200, 400, 800, and 1600...

Obviously higher ISO = noise.

BUT the E500 from the year before had a starting ISO of 50. I wonder if Olympus decided that ISO 100 was better for thier own cameras. Cause thats what I shoot at (as well as 200) and I never get a single grain of noise (except in really pitch black shots where my shutter speed is a little too fast.)

Thanks for posting this Bob - I've actually been thinking about it recently when it comes to testing cameras...

I know the D300 has a 'base' ISO of 200, and that's why one EV below isn't called 100, but L1.0. Similarly, the 50 ISO setting on the Canon full framers is actually referred to as Lo. As Bob's graph implies, the base sensitivity of these models is actually 100 ISO.

Normally the lowest selectable ISO yields the least noise and best quality, but as Bob points out, that's not necessarily the case.

The question is at what ISO should we test and compare cameras? For the outdoor and studio resolution pages, I always use the lowest selectable ISO, so for the D300 that was L1.0 / 100 ISO.

Should it instead have been tested at its base ISO of 200, and if so, should the comparison be against a camera also at 200 ISO, or at its own base sensitivity, which could be 100 ISO.

As you can see, you can quickly run into compromise and there's no ideal solution. Of course we do also have samples at every ISO on the real life nosie page, so I'm not too worried, but if anyone has any strong feelings about the methodology used on the resolution pages, do let me know!

Gordon

PS - and no, I don't know the base ISO for all cameras! I'd assume that unless they state otherwise, it is the lowest numerical ISO, but feel free to prove me worng.

The question about testing methodology is interesting, isn't it? Setting aside all the technological jumbo-mumbo it would only seem fair when demonstrating best case pictures to compare shots at 'base' ISO, so a D300 picture shot at ISO 200 should be compared against, say, a 40D picture at ISO 100. To do anything else puts, in this example, the D300 at a disadvantage. Logically it also follows that a D300 ISO 400 shot may most naturally be compared to a 40D ISO 200 shot as both camera read amplifiers are being set to a gain of "times two" but that may be a step too far for most people.

Maybe the most easily digestible solution is to continue as you do now with comparison shots ordered with each row containing the same ISO and including some text mentioning the 'base' ISO (maybe including a link to a more accessible feature than my post above explaining why 'base' ISO matters). For the Gallery shots it might be an idea to shoot at 'base' ISO, but again I think you would have to add a little explanatory text, possibly with an appropriate quote from the manual, explaining why. Have the manufacturers expressed any preferences to the reviewers over this?

Quickly returning to the mumbo-jumbo, it seems to me that a manufacturer can increase the native ISO of a sensor in both good and bad ways. The bad way is to reduce the full-well capacity. The good way is to increase the quantum efficiency of the detector and/or to increase the effectiveness of the microlenses. If the good way is followed, as Nikon appears to have done with the D3 and D300, then that should be applauded in the reviews. I can't think of a downside as you can always stop down or add ND filters rather than go below the 'base' ISO.